A stapler generally binds the bundle of sheets of paper to be bound by passing the legs of the staple through the bundle of the sheets of paper and bending them but a thickness of the bundle of the sheets of paper to be bound varies according to number of the sheets and is not fixed. Therefore, if some sheets of paper to be bound are bound by the staple and 40 through 50 sheets of paper to be bound are bound by the same staple, lengths of legs of the staples appeared on the back side of the bundle of sheets of paper when the legs of the staple pass through the bundle of sheets of paper are different from each other. The legs of the staple appeared on the back side of the bundle of sheets of paper are long when the bundle of the sheets of paper to be bound is thin but the legs of the staple appeared on the back side of the bundle of sheets of paper are short when the bundle of the sheets of paper to be bound is thick. However, when the legs of the staple appeared on the back side of the bundle of sheets of paper are long, it looks poor when being bound. It is preferable that the lengths of the legs of the staples projected from the back side of the bundle of the sheets of paper to be bound are as identical as possible.
As means for coping with this, a method of preparing some cartridges respectively loading the staples with legs having different lengths and using different cartridges for different thicknesses of the bundle of the sheets of paper to be bound and a method of cutting the too long legs of the staple when the bundle of the sheets of paper to be bound is thin (see Japanese Patent Application Publication No. 2005-119246) have been known. Particularly, in the stapler mounted on an expensive high-speed copy machine, it is necessary to provide a cutting mechanism for making uniform the lengths of legs of the staples projected from a back side of the bundle of the sheets of paper to be bound.